1. Field of the Invention
The present invention relates to an improved method for the production of vanadium/phosphorus mixed oxide catalysts which have special utility for the production of maleic anhydride.
2. Description of the Prior Art
A great deal of work has been done in the preparation and use of vanadium phosphorus oxide catalysts for the production of maleic anhydride. See U.S. Pat. Nos. 5,137,860 and 5,364,824 for a comprehensive description of the efforts of prior workers in this area.
Efforts have been made to develop catalyst preparation procedures which do not involve the use of corrosive and hazardous reagents such as HCl and oxalic acid.
U.S. Pat. No. 4,517,371 for example, at column 4, lines 48-64 summarizes prior work as follows:                “Generalizing from the above discussion, conventional preoperative methods, including both the aqueous and organic solution techniques, are unsatisfactory in that:        (1) they usually require that the catalyst manufacturing equipment be fabricated of special corrosion-resistant materials of construction;        (2) they are troubled by serious waste-disposal problems arising out of the employment of hydrogen chloride, nitric acid or oxalic acid for the dissolution of the vanadium component;        (3) they generally require extended and complex procedures for activation of the precursor catalyst;        (4) the preparation of the precursor catalyst is generally complicated and inherently costly; and        (5) the aqueous-based preparations result in catalysts of relatively poor activity and yield for converting butane to maleic anhydride.”        
The process provided by U.S. Pat. No. 4,517,371 is described at column 5, lines 18-43 as follows:                “In one aspect of the present invention there is provided a process for preparing a composition comprising vanadium, phosphorus and oxygen capable of catalyzing the oxidation of hydrocarbons comprising:        1. reacting a vanadium containing compound and a phosphorus containing compound in the presence of a liquid organic media in a manner and under conditions sufficient to form in said liquid organic media a heterogeneous, vanadium-phosphorus-oxygen first catalyst precursor composition having an atomic ratio of phosphorus to vanadium of from about 0.5:1 to about 2:1, and an average vanadium valence of from about 3.9 to about 4.7;        2. separating said first catalyst precursor composition with from said liquid organic media;        3. contacting said first catalyst precursor composition with at least one part by weight water per part by weight first catalyst precursor composition at a temperature of at least 30° C. to form a second vanadium-phosphorus-oxygen catalyst precursor composition;        4. separating said second catalyst precursor composition from said water; and        5. activating said second catalyst precursor composition.”        
Takita et al in an article entitled “Incorporation of promoter elements into the crystal lattice of (VO)2P2O7 and its promotion effects on the oxidation of n-butane to maleic anhydride”, Applied Catalysis A: General, Vol 103, pages 281-290 (1993) describe preparing a homogeneous solution of reduced and dissolved V2O5 in isobutanol was added V2O5 to isobutanol and refluxing for 10 hours, then adding 99% H3PO4. A solution of metal acetoacetonates in isobutanol was added and reflux continued for 1 hour. A small amount of water was added and a precipitate was formed and separated.
Our earlier U.S. Pat. No. 5,922,637 describes digesting a vanadium+5 compound in an organic solvent with added phosphorus compound at anhydrous conditions and later adding water and further digesting the material to obtain the catalyst precursor.
Our earlier U.S. Pat. No. 5,885,919 employs an additive such as dimethyl sulfoxide to produce an improved catalyst.
U.S. Pat. No. 3,975,300 employs a one-step procedure for the catalyst preparation in the absence of hydrogen halide by forming a paste of a vanadium compound, an organic reducing agent such as a glycol and a phosphorous compound and drying and calcining the paste.
U.S. Pat. Nos. 4,064,070, 4,132,670 and 6,174,833 discuss the use of a mixture of IBA/benzyl alcohol for the reduction of the V205. In all these cases there is a pre-reduction step of the vanadium pentoxide before the addition of phosphoric acid. This pre-reduction step is a separate reflux step of 3-5 hours. The 2nd reflux starts after the addition of the phosphoric acid and takes between 3-20 hours. In this step the formation of the VPO precursor takes place. In the present invention there is a single reflux step without the need of a pre-reduction step.
U.S. Pat. No. 5,155,235 the synthesis is anhydrous with no chloride in which the V2O5 is first reduced in a solvent containing 60% IBA and 40% benzyl alcohol (2 hr reflux). Thereafter the reaction mixture is cooled and the promote acetylacetonat magnesium or Zr is added followed by the addition reflux. The product recovery is by filtration of the cool solution (note catalyst calcination is at 500° C./3 hr under nitrogen).
U.S. Pat. No. 5,137,860 this patent provides an example of chloride free synthesis in which the V2O5 was reduced by oxalic acid in IBA. The main focus of the patent is on catalyst activation to form an activated phase with air/nitrogen/steam.
According to this patent, the synthesis procedure consists of a reflux step of a reaction mixture containing V2O5 in IBA, oxalic acid and phosphoric acid. After 16 hours of reflux, 25% of the IBA present is stripped followed by decantation of the remainder of the IBA and drying of the residual slurry.
U.S. Pat No. 5,364,824 is similar to U.S. Pat. No. 5,137,860 but it also shows the use of Bi as a promoter in a synthesis also with oxalic acid and IBA. The promoter was added after an overnight reflux step with phosphoric acid and cool down followed by another reflux step. This was referred as RCR (reflux cool down reflux).
The present invention provides a still further improved process for the preparation of VPO catalysts having special utility in the oxidation of C4 hydrocarbons to form maleic anhydride.